Experiment: Redox titration;determination of ascorbic acid by an iodometric back-titration
Why is sodium carbonate added to the thiosulfate solution ?
why is the water used for preparing the thiosulfate solution boiled ?
You want the pH to be about 9 and Na2CO3 will do that. It aids in preventing decomposition of the thiosulfate.
To understand why sodium carbonate is added to the thiosulfate solution in a redox titration for the determination of ascorbic acid, we need to consider the chemical reactions involved in the process.
Sodium carbonate is added to the thiosulfate solution as a means of eliminating any acidic impurities that could interfere with the titration reaction. The addition of sodium carbonate helps to neutralize any free acids that may be present in the solution. This is important because acidic conditions can influence the redox reaction and introduce errors in the titration.
As for why the water used for preparing the thiosulfate solution is boiled, the purpose is to remove any dissolved gases, particularly dissolved oxygen. Oxygen can react with thiosulfate ions, oxidizing them to form sulfate ions prematurely. By boiling the water, the dissolved oxygen is driven off, minimizing any potential interference with the redox reaction during titration.
In summary, sodium carbonate is added to the thiosulfate solution to neutralize acidic impurities that could affect the redox titration, while boiling the water used for preparing the thiosulfate solution removes dissolved gases, particularly oxygen, which could interfere with the reaction.